diff --git a/third_party/agg23/agg_vcgen_dash.cpp b/third_party/agg23/agg_vcgen_dash.cpp
index f690760b0..d44fca178 100644
--- a/third_party/agg23/agg_vcgen_dash.cpp
+++ b/third_party/agg23/agg_vcgen_dash.cpp
@@ -18,8 +18,11 @@
 //
 //----------------------------------------------------------------------------
 
+#include <cmath>
+
 #include "agg_shorten_path.h"
 #include "agg_vcgen_dash.h"
+#include "third_party/base/check_op.h"
 
 namespace pdfium
 {
@@ -60,6 +63,8 @@ void vcgen_dash::dash_start(float ds)
 }
 void vcgen_dash::calc_dash_start(float ds)
 {
+    DCHECK_GT(m_total_dash_len, 0);
+    ds -= floor(ds / m_total_dash_len) * m_total_dash_len;
     m_curr_dash = 0;
     m_curr_dash_start = 0;
     while(ds > 0) {